Several trends presently exist with regards to wireless communication devices. For example, in comparison to previous generations of wireless devices, modern wireless devices are more compact, more affordable, and have longer battery lifetimes. Major changes are occurring within the wireless industry in regard to transceivers and how they interface with industry-standard products, such as GSM, UMTS, LTE, WiMax, WLAN, Bluetooth, GPS, DVB, WiBree and so on. For example, baseband products have historically used analog I and Q signals in commercial and proprietary digital cellular systems, although digital interfaces are emerging more as the standard.
Not only are digital interfaces bringing benefits, but changes in system partitioning are enabled by digital interfaces. This lowers cost and accelerates time-to-market. Digital signal processing (DSP) techniques are commonplace in many digital applications and are used within baseband processors also. By converting from RF to digital within the radio, DSP techniques can improve radio performance by supporting the implementation of FIR filters for anti-aliasing, antidroop, channel filtering, notch filter, digital modulation, etc.
Unlike analog basebands, purely digital basebands can take advantage of semiconductor process density improvements, which are achieved much more rapidly in digital technology compared to analog. For example, in highly integrated GSM transceivers the die area can be dominated by analog passive components. The area required for a given capacitance has been reduced over the years, but not like the doubling of density described by Moore's law for digital functions.
The latest mobile phones provide multiband and multimode operation on cellular networks. The number of communication pipes for Wi-Fi connections, digital TV, digital audio broadcast and GPS satellite reception, among other technologies continue to increase. In recent years, one way in which designers have tried to deliver compact and efficient chipsets is by including zero intermediate frequency (IF) receivers. A zero IF receiver enables direct conversion of analog radio frequency (FREQ) signals to a digital baseband. This typically reduces the component count, and may correspondingly limit the footprint and cost of the chipset. By reducing the number of components, zero IF receivers also simplify the supply chain and improve manufacturing yield.
While zero-IF receivers offer a more compact chipset, technical barriers often limit the extent to which such receivers can be used in modern communication systems. For example, because a local oscillator signal (LO) in these receivers is the same as the RF frequency, the LO signal may leak from the receiver to the antenna, which can cause interference with other receivers on the same frequency-band. Also, DC offset, which comes from the self-mixing of LO leakage, may seriously deteriorate the SNR (Signal Noise Ratio).
One type of receiver that limits both of these shortcomings (i.e., LO leakage and DC offset) is a low-IF receiver. In low-IF receivers, the received RF frequency is down-converted to a low, but non-zero IF, before being down-converted to the baseband. Thus, the down-conversion from the received RF frequency to the baseband will have one or more IFs, where each IF corresponds to a separate stage in the receiver. Due to the fact that these separate stages are relatively area intensive, conventional low-IF receivers have a relatively large footprint.
Convergence among mobile devices means that many combinations of these RF communication/broadcast standards will appear in PDA's, laptop computers and game consoles. In these products, space, cost and power consumption constraints will make it no longer viable to have a dedicated wireless transceiver for each standard. Therefore, there is a need for partitioning interfaces for these devices capable of handling various standards while also avoiding overhead in terms of cost and current on the platform.